Cesium-137, strontium-90, and actinides account for a significant amount of the radioactivity of liquid wastes, such as high level liquid wastes from nuclear fuel reprocessing. Cesium-137 and strontium-90 account for over 99.9% of the relative toxicity of the liquid waste once the actinides have been removed. Cesium-137 has a halflife (“t1/2”) of 30 years and strontium-90 has a t1/2 of 29 years. This liquid waste is extremely hazardous and expensive to dispose of. To increase safe handling of the majority of the liquid waste and to significantly reduce its storage and disposal cost, the liquid waste is separated into two portions: one containing the majority of the radioactive components and one containing the bulk of the non-radioactive components. Removing the radioactive components allows the liquid waste to be decategorized and disposed of in geological formations after vitrification. Currently, separate technologies are used to remove the actinides and fission products from the liquid waste and, often times, separate processes are used to remove specific radionuclides, such as cesium and strontium.
The ability to remove and recover cesium and strontium from spent nuclear fuel waste represents a significant issue regarding short term heat loading in a geological repository. Cesium and strontium are major heat generators in the liquid waste and produce gamma and beta radiation. Removing the cesium-137 and strontium-90 would enable these radionuclides to be stored in a short-term waste facility, enabling long-term storage facilities to store waste closer together by eliminating some of the heat load.
Liquid extraction, sorption, and coprecipitation methods have been used to remove cesium or strontium from nuclear acidic waste solutions or related alkaline wastes. Numerous extractants have been identified that extract cesium or strontium from alkaline solutions or acidic solutions. The extractants are typically separate solvents that are designed to remove one of these radionuclides. For instance, crown ether compounds or calixarene crown ether compounds have been used to extract cesium. U.S. Pat. No. 6,174,503 to Moyer et al., U.S. Pat. No. 6,566,561 to Bonnesen et al., Duchemin et al., Solvent Extr. And Ion Exch., 19(6):1037-1058 (2001), Leonard et al., Solvent Extr. And Ion Exch., 21(4):505-526 (2003), Leonard et al., Sep. Sci. and Technol., 36(5-6):743-766 (2001), White et al., Sep. Sci. and Technol., 38(12-13):2667-2683 (2003), and Norato et al., Sep. Sci. and Technol., 38(12-13):2647-2666 (2003) disclose extracting cesium from alkaline solutions using calix[4]arene-crown ether compounds. The calix[4]arene-crown ether compounds and modifiers are dissolved in a diluent. The calixarene is calix[4]arene-bis-(tert-octylbenzo)-crown-6 (“BOBCalixC6”). Strontium is removed from the alkaline solutions in a separate process using monosodium titanate. One specific extractant includes 0.007M BOBCalixC6, 0.750M 1-(2,2,3,3-tetrafluoro-propoxy)-3-(4-sec-butylphenoy)-2-propanol (“Cs-7SB”), 0.003 M trioctylamine (“TOA”), and Isopar® L and is referred to herein as the caustic-side solvent extraction (“CSSX”) solvent. The CSSX solvent provides a forward distribution ratio or coefficient for cesium (“DCs”) of 8.0 from a 1M nitric acid solution. Another specific extractant includes 0.01M BOBCalixC6, 0.5M Cs-7SB, 0.001 M TOA, and Isopar® L.
U.S. Pat. No. 5,926,687 to Dozol et al. and Bonnesen et al., “Development of Process Chemistry for the Removal of Cesium from Acidic Nuclear Waste by Calix[4]arene-crown-6 ethers,” ACS Sym. Ser. 757 (Calixarenes for Separations), 26-44 (2000) disclose extracting cesium from acidic solutions using calix[4]arene-crown ether compounds. While the tested calix[4]arene-crown ether compounds have high distribution coefficients for cesium, they have low distribution coefficients for strontium. Various calix[4]arene-crown ether compounds and modifiers were tested because the stability of the calix[4]arene-crown ether compounds and modifiers differed in each of these solutions. In Dozol et al., Sep. Sci. and Technol., 34(6&7):877-909 (1999), monocrown or biscrown calix[4]arenes in a 1,3 alternative cone conformation are disclosed to remove cesium from acidic or alkaline solutions.
U.S. Pat. No. 5,888,398 to Dietz et al. discloses using an 18-crown-6-ether to extract cesium from acidic solutions. The 18-crown-6-ether selectively extracts cesium over other ions, such as hydrogen, aluminum, calcium, boron, and strontium.
U.S. Pat. Nos. 5,344,623 and 5,346,618 to Horwitz et al., U.S. Pat. No. 6,511,603 to Dietz et al., Lamb et al., “Novel Solvent System for Metal Ion Separation: Improved Solvent Extraction of Strontium(II) and Lead (II) as Dicyclohexano-18-crown-6 Complexes,” Sep. Sci. and Technol., 34(13):2583-2599 (1999), Chiarizia et al., “Composition of the Organic Phase Species in the Synergistic Extraction of Sr2+ by Mixtures of Di(2-Ethylhexyl)Alkylenediphosphonic Acids and Dicyclohexano-18-crown-6,” Solvent Extr. And Ion Exch., 21(2):171-197 (2003), and Tanigawa et al., Chem. Eng. J. 39:157-168 (1988) disclose extracting strontium from an acidic solution using crown ethers. One specific extractant includes a mixture of 0.15M 4′,4′,(5′)-di-(t-butyldicyclo-hexano)-18-crown-6 (“DtBu18C6”) and 1.2M tri-n-butyl phosphate (“TBP”) in Isopar®L and is referred to herein as the strontium extraction (“SREX”) solvent, as described in Horowitz et al., Solvent Extr. And Ion Exch., 9(1):1-25 (1991). The SREX solvent provides a distribution ratio or coefficient for strontium (“DSr”) of 0.7 from a 1M nitric acid solution.
However, using separate extractants to remove the cesium and strontium is disadvantageous in regard to environmental concerns, safety, simplicity and effectiveness of processing, and undesirable generation of secondary waste.
Methods of extracting both cesium and strontium have also been disclosed. In U.S. Pat. No. 4,749,518 to Davis et al., cesium is extracted from acidified nuclear waste with bis 4,4′(5) [1-hydroxy-2-ethylhexyl]benzo 18-crown-6 and a cation exchanger. The strontium is then extracted using bis 4,4′(5′) [1-hydroxyheptyl]cyclohexo 18-crown-6 and a cation exchanger. In U.S. Pat. No. 5,393,892 to Krakowiak et al., a method of removing alkali metal and alkaline earth metals is disclosed. A solid inorganic support having a ligand covalently bonded thereto is contacted with a solution including the alkali metal and alkaline earth metals. The ligand is an oxygen donor macrocyclic polyether cryptand that selectively removes the alkali metal and alkaline earth metals. In U.S. Pat. No. 5,666,641 to Abney et al., a polymeric material including a polymer and a plasticizer is used to extract cesium and strontium. In U.S. Pat. No. 5,666,642 to Hawthorne et al., metal dicarbollide ion complexes are used to remove cesium and strontium from an aqueous fission product waste solution. The metal dicarbollide ion complexes are used to sequentially remove the cesium and then the strontium. In Horwitz et al., International Solvent Extraction Committee '96, “A Combined Cesium-Strontium Extraction/Recovery Process,” p. 1285-1290 (1996), an extraction process using di-t-butylcyclohexano-18-crown-6 and a macrocyclic polyether are disclosed to simultaneously extract cesium and strontium.
In addition, a large scale demonstration of concurrent cesium and strontium partitioning from defense-related nuclear waste was performed in Russia using a cobalt dicarbollide extraction process. In U.S. Pat. No. 6,270,737 to Zaitsev et al., a composition of a complex organoboron compound and polyethylene glycol in an organofluorane diluent is used to extract cesium and strontium. The complex organoboron compound is a halogenated cobalt dicarbollide. In U.S. Pat. No. 6,258,333 to Romanovskiy et al., a composition of a complex organoboron compound, polyethylene glycol, and a neutral organophosphorus compound in a diluent is used to simultaneously extract cesium and strontium. The complex organoboron compound is a halogenated cobalt dicarbollide. However, this extraction process uses multiple chemicals and, therefore, adds significant volume to the waste volume produced by the extraction process.
It is desirable to develop an extraction process that simultaneously removes or extracts cesium and strontium from an acidic solution. Such a development would improve capacity of long-term storage facilities and reduce the need to create new storage facilities. In order to be useful in large-scale processing applications, the solvent used in such an extraction process would desirably be highly selective, cost effective, produce reduced waste volume, and be relatively nonhazardous.